1. Field of the Invention
The present invention relates to a method for producing rare earth-containing permanent magnets which are highly corrosion resistant, and in particular to a method for producing sintered rare earth-iron-boron-based permanent magnets the surfaces of which are coated uniformly with a corrosion resistant metal layer.
2. Description of the Prior Art
Owing to their excellent magnetic properties and inexpensiveness, rare earth permanent magnets are extensively used in the electric and electronic industrial fields. The ever progressing technology in these fields constantly demands further improvements in the performances of these magnets. Rare earth permanent magnets containing neodymium as a rare earth element are especially favored and are replacing the samarium-cobalt-based rare earth permanent magnets in the small-type magnetic circuits. This is because the magnetic properties of the neodymium-containing rare earth permanent magnets are far better than those of the conventional Sm-Co-based rare earth permanent magnets, neodymium is naturally more abundant than samarium, and the neodymium-containing rare earth magnets require much less expensive cobalt component as compared to the conventional Sm-Co-based rare earth permanent magnets. Also, the economy of the neodymium-containing rare earth magnets has motivated their use in the various applications where hard ferrite and alnico magnets or electromagnets are conventionally used. However, like all of the other rare earth elements, neodymium has an unfavorable tendency to easily oxidize in air, and especially in moist air. This oxidation not only gives rise to an oxide layer in the surfaces of the magnet, but also proceeds inwardly to cause intergranular corrosion, which develops along the grain boundary. This phenomenon is the most noticeable in the Nd magnets, because a very active Nd-rich phase exists in the grain boundary of the Nd magnets. The intergranular corrosion leads to a profound decrease in the magnetic properties, and if the corrosion progresses while the magnet is in use, the performance of apparatus using the magnet deteriorates, and the peripheral devices are contaminated.
Various surface treatment methods have been proposed to solve the oxidation problem of the rare earth magnets, and particularly, the neodymium-containing magnets. However, none of the proposed methods has been sufficient to put an end to the problem. For example, a method whereby the magnet surfaces are coated by spraying or electrocoating with a resin film results in rusting immediately beneath the resin film due to the hygroscopicity of the resin. Vapor plating methods, such as vacuum deposition, ion spattering, and ion plating, are costly and are not effective in coating the receded surfaces, such as the holes and grooves.